1. Field of the Invention
This invention relates to electronic ballast systems for gas discharge tubes. In particular, this invention pertains to electronic ballast systems for fluorescent type gas discharge tubes. In particular, this invention relates to an electronic ballast system which is frequency stabilized. More in particular, this invention pertains to an electronic ballast system which has an automatic gain control circuit. Still further, this invention relates to an electronic ballast system which provides for a frequency stabilized automatic gain controlled network and provides for a minimization of electrical components in combination.
2. Prior Art
Electronic ballast systems for gas discharge tubes and for fluorescent tubes in particular are known in the art. Additionally, ballast systems for a plurality of fluorescent discharge tubes is also known in the art.
However, in many prior art ballast systems, the gain of included switching circuits must be adjusted or matched which results in the addition of components such as potentiometers or additional electronic circuits to maintain substantially equal outputs of component power transistors irrespective of their individual current gain. Addition of such elements add to the overall volume and cost of such prior art ballast systems.
Additionally, in many prior art ballast systems which operate at more than double the line frequency, the operating frequency is not constant and varies with supply voltage or load current, or possibly with both. Such prior art systems due to this frequency variability may cause a flicker effect when one of a multiplicity of fluorescent lamps is electrically removed from the overall circuit. In some cases, where one of the fluorescent lamps is removed from the circuit, a second fluorescent tube may be entirely extinguished.
Many such prior art systems use the commonly known Jensen Circuit (I.R.E. Transactions on Circuit Theory, September 1957, James Lee Jensen) where the frequency of oscillation generated by a saturable transformer is dependent upon the magnitude of the voltage feedback from a non-saturating power transformer through a series resistor. This type of system operates in such a way that one transistor is in an "on" state with its collector approximately at the potential of the emitter, and the other transistor is in an "off" state with its collector at approximately twice the supply voltage. Thus, the voltage applied to the saturating transformer will be equal to twice the supply voltage minus the drop across the series resistor. As this transformer goes into saturation, the current through the series resistor will increase, thereby increasing the voltage drop across the resistor and having the effect of decreasing the power supplied to the saturating transformer and decreasing the loop gain as well. This sequence of events causes the transistors to change state. The base drive of the "on" transistor becomes insufficient for it to remain in conduction and induced voltages will decrease. Voltages of opposite polarity appear and the base drive of the previously "off" transistor will increase turning it to an "on" state and re-establishing the sequence of events to make a recurring cycle. Hence, it can be seen that this type of prior art system has an operating frequency dependent on when the saturating transformer goes into saturation and such varies with supply voltage or changes in the load which effect the feedback voltage.
Additionally, in such prior art ballast systems, the shift in frequency caused by the change in load current when a fluorescent lamp is electrically removed from the circuit may cause high voltage surges which are destructive to such systems. Thus, in such prior art systems, there must be incorporated additional electrical components to prevent damage to such prior art circuits. The addition of electrical components in these prior art systems increases the volume required by such systems and further increases the heat dissipation of the prior art ballasts.